1. Technical Field
The present invention relates to a tractor which detachably mounts a hydraulic circuit body and a hydraulic control valve on a transmission casing.
2 . Background Art
Conventionally, as a type of tractor, there has been known a tractor in which a hydraulic casing is communicably connected with an upper portion of a transmission casing, a lift arm support shaft which has an axis thereof extended in the lateral direction penetrates the hydraulic casing, proximal end portions of a pair of left and right lift arms are mounted on left and right-side end portions of the lift arm support shaft, a proximal end portion of an operating arm is mounted on a midst portion of the lift arm support shaft, a distal end portion of a lift cylinder which is arranged in the inside of the hydraulic casing in a lying manner is connected to a distal end portion of the operating arm, and a lift arm is vertically and rotatably operated in an interlocking manner with the extending and contracting operation of the lift cylinder (for example, see Japanese Patent Laid-open 2002-283861).
Further, a hydraulic circuit is formed in the hydraulic casing, and a lubricant in the inside of the transmission casing is allowed to pass through the hydraulic circuit as a working oil so as to perform the extension-contraction operation of the lift cylinder.
However, with respect to the above-mentioned tractor, since the hydraulic casing is often formed by casting and hence, there may be a case that small cavities, that is, so-called blowholes are formed in the hydraulic casing. When the hydraulic circuit is formed in such a hydraulic casing, there may arise a drawback that the pressurized oil leaks at blowhole portions.
This brings about a drawback that the whole hydraulic casing is scrapped or disposed and hence, an economical loss is large.
Further, in the above-mentioned tractor, the lubricant in the inside of the transmission casing is supplied to the lift cylinder under pressure as a working oil so as to operate the lift cylinder. Accordingly, when a hydraulic filter is clogged with dusts such as worn-out pieces which are constantly generated due to the meshing of gears in the inside of the transmission casing, the dusts flow into a hydraulic control valve formed in the midst of the hydraulic circuit through a bypass circuit and hence, there exists the possibility that the operation of the hydraulic control valve becomes defective.
Accordingly, there has been adopted the structure in which a partition plate is interposed between the hydraulic casing and the transmission casing so as to separate the working oil in the inside of the hydraulic casing and the lubricant in the inside of the transmission casing.
However, such separation structure suffers from following drawbacks.
(1) In performing the performance/function inspection of the hydraulic control valve, to prevent the leaking of the working oil, it is necessary to perform the inspection in a state that the hydraulic casing is mounted on the transmission casing. Accordingly, the inspection device becomes large-sized and there exists the cumbersomeness that a human power is insufficient for handling the inspection device and a transport assist device is necessary.
(2) It is necessary to prevent the leaking of oil on both surfaces of the partition plate and hence, the highly accurate flatness is required with respect to both surfaces of the partition plate.
(3) Since the partition plate is interposed between the hydraulic casing and the transmission casing, a height on the ground of the hydraulic casing is increased by an amount corresponding to a wall thickness of the partition plate whereby the degree of freedom in designing a mounting height of a driver's seat which is arranged right above the hydraulic casing is small.